According to the traditional position of the ATF, converting to a semi does not make it not a machine gun, therefore you would need to transfer on the appropriate form.

On the other hand, I heard a story about an AR type being taken off the registry because it was semi-only.

The obvious question is why you would want to do this. A transferable machine gun is typically worth at least an order of magnitude more than the same gun in semi auto because of the market restrictions.

Even if what you proposed could be done, your customer couldn't switch it back to full auto.